[p5.js 2.0 Docs] Change in return value for load functions that use callbacks needs to be documented

Topic

In version 1.x, the load... functions (loadImage, loadJSON, etc.) always returned the loaded data, whether or not callbacks were used. This changed in version 2: now when load.. functions are used with callbacks, they now return the value returned by the callback instead of the loaded item. This is a nice feature that allows a successCallback to dynamically modify the loaded data. It also allows an errorCallback to return some default data. But this feature needs to be described in the documentation for the various load... functions. The need for this was noted by @limzykenneth in a comment on a previous issue, but it looks like it was missed.

This is a breaking change from version 1 to 2 since most version 1 sketches that use both an assignment and a callback function won't return the desired data from the callback function (this has actually broken one of the examples on the p5.js website; see /processing/p5.js-website#1033), so this should be emphasized in the documentation.

Review the documentation entries for the load... functions, starting with loadImage and loadJSON and their existing return-value sections. Document that callback-based calls return the callback's value, including success and error callbacks, and emphasize the v1-to-v2 breaking change. Done when all relevant load-function references consistently describe this behavior.